Hi decbuck,
afaik a plc is a picece of equipemnt used to automate processes in factories. The people who specify, configure and install them are usually Electrical Engineers, Technicians and Electricians although I know some Mechanical Engineers who are pretty familiar with them as well. The companies who manufacture import and distribute them also employ technical and sales people.
ajapale
PLC's are used in a manufacturing process, they are programmed with a special language and there are a variety of manufacturers of them, hence a variety of languages.
Like anything, The right people are the well paid!
I would only study them if my job involved them, otherwise I wouldn;t bother until then, because you really need to be using them to become familiar with them, again like anything!